The present invention concerns a process for removing chromium from chromium containing liquids. In particular, it concerns removing chromium from aqueous alkali metal or alkaline earth metal sulfate solutions. More particularly, it concerns the removal of alkali metal dichromates from aqueous solutions of alkali metal sulfates by reacting said solutions with inorganic sulfur containing compounds. Most particularly, the present invention is concerned with the removal of sodium dichromate from sodium sulfate solutions.
Sodium sulfate is produced as a by-product in certain industrial processes. When it is contaminated with chromium because of exposure to chromium containing catalysts or otherwise, it is desirable to remove the chromium prior to use of the sodium sulfate, for instance, in the glass or textile industry or as a component of a detergent.
Kaczur et al., U.S. Pat. No. 4,259,297, discloses a multistep process for removing hexavalent chromium from impure aqueous alkali metal chlorate solutions. The pH of the impure aqueous solution is adjusted in a first step to a first intermediate pH in the range from about 9 to about 13 before mixing with the reducing chemical. An inorganic sulfur compound is then added. Then, in a second intermediate pH adjustment step in which the pH is adjusted from about 2 to about 4, the inorganic sulfur compound reduces the chromium to trivalent and divalent chromium. Finally, in a third intermediate pH adjustment step, the pH is adjusted from about 6 to about 8 and the reduced chromium is precipitated as the hydroxide.
Japanese Patent 119493 (1980) discloses a multistep method of removing low levels of chromium (i.e., 0.5 to 50 ppm) from reconstituted chlorate solutions from crystallizers. In this multistep process, hexavalent chromium is reduced by adding sulfite at a pH of from 3 to 6. The amount of sulfite added must satisfy a complex equation which is a function of pH and concentrations. The patent then states that the pH is adjusted from 9 to 11 to precipitate the chromium as hydroxide, but no details are provided as to how this can be accomplished.
Both of the above-described references produce chromium hydroxides which are well known to be difficult to filter. Kaczur discloses an expensive and awkward multistep solid-liquid separation process employing two centrifuging stages and two filtration stages. Japanese Patent 119493 does not address the solid-liquid separation problem. No commercial process exists employing either of these two processes.
U.S. Pat. No. 4,376,099 to Yamamoto et al. disclose in column 2, lines 3-9 the formation of a precipitate of hydrous chromic oxide subsequent to the acidification of a chlorate cell liquor containing hexavalent chromium ions by passing the chlorate cell liquor through an anion exchange resin bed. There is no indication of the use of a sulfur compound under neutral or acidic conditions as a reducing agent which is used to treat a chromium ion containing aqueous liquid so as to precipitate chromium oxide.
Japanese Patent 9027057, Teikoku Piston Ring Company, discloses in the abstract a process for the treatment of waste water containing hexavalent chromium ions so as to remove the hexavalent chromium by treatment with sulfur dioxide in a gas-liquid contactor. The abstract further discloses the treatment of waste water having a pH of 6.6 and containing 98 parts per million of hexavalent chromium by adjustment of the pH to less than 4.2 with 10 percent sulfuric acid and, thereafter, treatment with 99.96 percent sulfur dioxide at 15 degrees centigrade in a counter-current packed column. The hexavalent chromium remaining in the treated waste water after neutralization with 20 percent sodium hydroxide was 0.05 parts per million. There is no indication that the method used to separate the hexavalent chromium subsequent to reaction with sulfur dioxide involves the removal of chromium as solid particles of chromium oxide.
In the present invention, there has been discovered a set of conditions which allow the chromium to be simultaneously reduced and precipitated as an easily filtered metal oxide in a single stage process. Because of the process conditions of the present invention, solid-liquid separation can be accomplished on a full scale basis in a single filtration step without the use of clarifiers or centrifuges.
The present invention offers many advantages: (1) There is no release of noxious by-products such as sulfur or chlorine based gases which are characteristics of most other chromium precipitation processes. (2). The sulfur based reducing agents are readily available and, when used in the alkaline solution form, are safe and easy to store and handle without the release of SO.sub.2. (3) There are no by-product reaction contaminants in the chromium free sulfate product. (4) The resulting chromium precipitate, which is mainly chromium oxide, is more easily filtered than the gelatinous chromium hydroxide precipitates produced by the prior art processes, and filtering can be done inexpensively in a single stage with a filter without the use of a centrifuge. The chromium oxide precipitate collects on the filter as a dense filter cake. In applications where the chromium is to be disposed of or taken to another site for reprocessing having the chromium sludge in the form of a dense filter cake minimizes the total volume of hazardous chemical which has to be handled. (5) The precipitate is in such a form as to be easily converted into a usable form, unlike prior art processes which do not yield chromium in a usable form. (6) There are no flocculating agents or by-product reaction contaminants such as elemental sulfur in the precipitated chromium sludge which would require further processing before the sludge is reused. (7) Unlike prior art processes which utilize several pH adjustment steps from extremely alkaline to extremely acidic for the reduction and precipitation of chromium, the present invention involves a pH in the acid range which is optionally followed by one pH adjustment to the alkaline range. The present invention allows essentially 100% of the chromium from a sulfate sludge product to be safely and inexpensively removed and recycled without the generation of by-product sulfur or chlorine based gases characteristic of prior art processes.